Can tell me how to get this weight paint option in Blender 2.8 (screenshots inside)
You need to enable Vertex Selection Masking For Painting, then you can only paint the selected vertices. Now maybe I miss something but it looks like, unlike 2.7,it doesn't display any vertex, selected or not, so maybe the best way is to Show Wire in the Viewport Overlays panel, it still won't show any vertex though...

So if I'm right, to do what you want to do, it may be easier to switch to Edit mode and to use the Weight value and Assign button under the Vertex Groups list?